1. Field of the Invention
The present invention relates to a marine propulsion machine, such as an outboard motor having an outboard internal combustion engine, or an outboard/inboard motor having an inboard internal combustion engine. More particularly, the present invention relates to a gear case assembly with a pressure-compensating function, for a marine propulsion machine.
2. Description of Background Art
A gear case, forming a lower part of a marine propulsion machine, has a gear chamber formed therein for housing a bevel gear mechanism, used for transmitting power from a drive shaft to a propeller shaft. The gear case contains sufficient lubricating oil in the gear chamber to lubricate bearings, supporting a drive shaft, and other members that need lubrication. The gear case is sealed in a liquid-tight fashion to prevent the seepage of water into the gear case.
The gear case is designed such that the gear chamber has the least necessary volume to construct the marine propulsion machine in lightweight construction and to reduce fluid resistance that acts on the gear case. Therefore, air in spaces extending above the gear chamber in the gear case is compressed to absorb increase in the volume of the lubricating oil due to thermal expansion when the lubricating oil is heated by heat generated by the gear mechanism housed in the gear chamber.
A gear case provided with a pressure-compensating structure disclosed in, for example, JP 53-24098 U for an outboard engine is provided with a bellows of rubber or the like defining an expansion chamber connected to the gear chamber by a small hole, and disposed in an upper part of a gear chamber to limit the rise of pressure in the gear case below an allowable upper limit.
The expansion chamber of the bellows of the pressure-compensating structure disclosed in this reference expands to absorb an increase in the volume of air contained in the gear case so that the pressure in the gear case may not rise beyond the allowable upper limit when air contained in the gear case expands due to heating by heat generated by stirring the lubricating oil by gears rotated at high speeds in the gear chamber of a limited volume when the outboard engine operates.
A space for installing the special bellows defining the expansion chamber cannot be easily secured in a narrow space extending over the gear case.
The special bellows increases component parts and assembling work, and needs additional work for forming the small hole connecting the expansion chamber of the bellows and the gear chamber, which increases the cost.
The present invention has been made in view of the foregoing problems, and it is therefore an object of the present invention to provide an inexpensive gear case assembly with a pressure-compensating function for a marine propulsion machine, having a reduced number of parts, and capable of reducing the time required for assembly thereof.